The present invention relates to a process for preparing bonded crystalline alumina-containing nodules characterized with improved properties such as better crush strength and increased attrition resistance.
Crystalline alumina-containing nodules are well known in the art for use as sorbents, catalyst supports and as catalysts per se. Principal components of nodules used in these applications include alumina per se, aluminosilicates, and mixtures of alumina with aluminosilicates.
Activated alumina particles prepared by a number of heretofore known processes and sometimes designated in the trade as "hard alumina", while often exhibiting good crush resistance, have not been entirely satisfactory in regard to other mechanical properties, especially regarding attrition resistance. In general, prior art techniques for improving attrition resistance of nodules of alpha alumina, gamma alumina and other types of alumina have resulted in sacrifice of important mechanical properties, typically including loss of crush strength, pore size, etc.
A number of aluminosilicates are available to the art for use as sorbents, catalysts, and catalyst carriers. An example is natural or synthetic mullite. U.S. Pat. No. 3,533,738 to Rundell et al. describes a process for preparing a synthetic crystalline mullite which is disclosed to be desirable as a base for various catalysts.
Other crystalline aluminosilicates known in the catalyst and sorbent arts include natural and synthetic zeolites. Chemically the zeolites can be represented in hydrated form by the following general formula: EQU M.sub.2/n O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O
wherein M is a cation which may be a metal in group I or II of the periodic table, a transition element of the periodic table, hydrogen, ammonium, or mixtures of these cations, n is the valence of the cation, x is normally in the range of 1 to 15 and y is a function of the degree of hydration.
Natural zeolites include levynite, erionite, faujasite, analcite, paulingite, noselite, ferriorite, heulandite, scolecite, stilbite, clinoptilolite, harmotome, phillipsite, brewsterite, flakite, datolite, chabazite, gmelinite, cancrinite, leucite, lazurite, scapolite, ptilolite, mesolite, mordenite, nepheline, natrolite, and sodalite.
Some representative synthetic crystalline aluminosilicate zeolites include Zeolite A, Zeolite X, Zeolite Y, Zeolite L, Zeolite O, Zeolite R, Zeolite S, Zeolite T, Zeolite Z, Zeolite F, Zeolite Q, and Zeolite B. For more complete formula representations for each of these synthetic crystalline zeolites see U.S. Pat. No. 3,140,251 to Charles J. Plank et al.
Type A zeolite has a silica to alumina mole ratio of 1.85 .+-. 0.5 illustration and is described in greater detail in U.S. Pat. No. 2,882,243 to Milton.
The various zeolites can be distinguised one from others by their respectively characteristic X-ray diffraction patterns which indicate different crystal lattice structures.
The faujasites constitute an important group of zeolites. The faujasite structure consists of a diamond type crystal lattice of cubo-octahedral units joined by pairs of six-membered oxygen rings. Faujasite has a well-ordered structure having all the aluminum atoms tetrahedrally coordinated with oxygen and one excess negative charge for each aluminum atom in the lattice. This charge is electrically balanced by a highly mobile cation.
The zeolites designated Zeolite X and Zeolite Y by the Linde Division of Union Carbide Corporation are commonly referred to as synthetic faujasites. Zeolite Y is described in U.S. Pat. No. 3,130,007 to Breck and is generally similar to Zeolite X described in U.S. Pat. No. 2,882,244 to Milton. The chemical formula given for Zeolite Y in U.S. Pat. No. 3,130,007 may be represented as follows: EQU 0.9 .+-. 0.2 Na.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O
where x has a value of greater that 3, e.g., up to about 6, and y may have a value as high as 9. Type X zeolites have a silica to alumina ratio of from 2 to 3, e.g., about 2.5. The ratio of silica to alumina affects important physical properties of faujasite. Synthetic faujasite having a silica to alumina mole ratio of more than 4.5 is of special interest in high temperature applications in that thermal stability thereof is greater than the stability typically exhibited by synthetic faujasite having a lower silica to alumina ratio. Type X and Type Y zeolites find use as components of hydrocarbon cracking catalysts.
In contrast to faujasite, mordenite has a silica to alumina mole ratio of about 7 to 11, usually about 9 to 10.
Substantial effort has been devoted heretofore to develop efficient methods for improving mechanical properties of various aluminosilicates. Naturally occurring clays, such as kaolin, attapulgite and bentonite, inorganic oxide sols, and inorganic oxide gels and cogels, have heretofore been proposed for binding particles of sorbents, catalyst supports, and catalysts, with binderless compositions also being known to the art. However, in general, these approaches have not been entirely satisfactory from the standpoints of good balance of mechanical properties and efficiency.
For examples of use of silica-containing cogels as binders or matrix material, see U.S. Pat. Nos. 3,329,628 to Gladrow et al. and 3,393,156 to Hansford.
Gladrow et al. in U.S. Pat. No. 3,326,818 describe a method of making catalyst compositions which includes dry mixing 51 to 95 wt. percent crystalline aluminosilicate zeolite and 5 to 49 wt. percent of a dry inorganic gel binding agent (permissively dry alumina gel) containing a peptizing agent. Thereafter, sufficient water is added to form a thick plastic mass which is subsequently dried to a composite product including at least 5 wt. percent of the binding agent.
U.S. Pat. No. 3,365,392 to Mitsche et al. describes a reforming catalyst having preferably less than 20 wt. percent of a finely divided crystalline aluminosilicate suspended in an alumina matrix and at least one active catalytic component.
U.S. Pat. No. 3,498,928 to Cho et al. describes addition of an organic solution of aluminum alkoxide to a granular mixture of a zeolite with oxides of copper and manganese in a method for ultimately preparing an oxidizing catalyst, resulting in forming a network binding structure of 2 to 6% aluminum oxide in the catalyst granules.
In U.S. Pat. No. 3,562,345, Mitsche describes a process for preparing a composition comprising about 60 to about 90 weight percent of an aluminosilicate containing alumina fixed in combination therewith. Briefly, the process includes heating in admixture with an alumina sol an aluminosilicate characterized by a silica/alumina mole ratio of from about 6 to about 12 and pore openings of from about 3 to about 8 angstroms, recovering an aluminosilicate--alumina sol product, gelling the sol product, and thereafter washing and drying the aluminosilicate-alumina gel product.
Haden et al., referring in U.S. Pat. No. 3,065,054 to prior art methods of producing synthetic crystalline type A zeolites (stated to be inherently in powdered form), state that binders such as colloidal clays or hydrous alumina are used in order to agglomerate the powdered zeolites or sorbents. According to the description, substantial quantities of binder, often 20 percent or more, are used in order to produce pellets of adequate resistance to attrition with attendant decreases in sorptive capacity.
U.S. Pat. No. 3,406,124 to Eastwood et al. describes a composite organic conversion catalyst containing an active crystalline aluminosilicate component and an alumina containing clay component which has been leached and neutralized. A method for preparing such catalysts is disclosed which includes leaching alumina from a clay matrix material, precipitating aluminum hydroxide on the clay, and thereafter mixing the treated clay with finely divided crystalline aluminosilicate. The patent teaches that the precipitated aluminum hydroxide subsequently acts as a binder for the clay and results in production of an attrition resistant composite product.
Chemical Abstracts, Vol. 60, reference 12696d, briefly discusses use of aluminum hydroxide or silicic acid as a binder for molecular sieves, the binder apparently being derived from an aqueous solution of a water soluble aluminate or silicate with specified derivatives of organic acids.
Chemical Abstracts, Vol. 66, reference 87197d, in abstracting French Patent 1,450,633, discloses preparation of molecular sieve aggregates by extruding a mixture of sieve particles &gt; 1 .mu. and a binder selected from activated Al.sub.2 O.sub.3, SiO.sub.2, clay or cement. Corresponding British Patent 1,130,639 adds that the alumina is active alumina obtained by partial dehydration of alumina trihydrate usually crushed to small grain sizes.
Chemical Abstracts, Vol. 56, reference 13794f, refers to mechanical stability problems in fluidized systems of molecular sieve catalysts bonded with clay minerals or a gel of Al(OH).sub.3 to form granules of 3-5 mm. diam. A spray process for making molecular sieve zeolites in granules of 30-600 .mu. diam. is proposed wherein binder clay is added.
Chemical Abstracts, Vol. 73, reference 7567, indicates that fabrication of molecular sieves using Al.sub.2 O.sub.3 or SiO.sub.2 as binder is reviewed in Fetle, Seifen, Anstrichm.